Crosstalk among hydrogen sulfide (H2S), nitric oxide (NO) and carbon monoxide (CO) in root-system development and its rhizosphere interactions: A gaseous interactome

Unraveling the potential of hydrogen sulfide as a signaling molecule for plant development and environmental stress responses: A state-of-the-art review

Over the past decade, a plethora of research has illuminated the multifaceted roles of hydrogen sulfide (H2S) in plant physiology. This gaseous molecule, endowed with signaling properties, plays a pivotal role in mitigating metal-induced oxidative stress and strengthening the plant's ability to withstand harsh environmental conditions. It fulfils several functions in regulating plant development while ameliorating the adverse impacts of environmental stressors. The intricate connections among nitric oxide (NO), hydrogen peroxide (H2O2), and hydrogen sulfide in plant signaling, along with their involvement in direct chemical processes, are contributory in facilitating post-translational modifications (PTMs) of proteins that target cysteine residues. Therefore, the present review offers a comprehensive overview of sulfur metabolic pathways regulated by hydrogen sulfide, alongside the advancements in understanding its biological activities in plant growth and development. Specifically, it centres on the physiological roles of H2S in responding to environmental stressors to explore the crucial significance of different exogenously administered hydrogen sulfide donors in mitigating the toxicity associated with heavy metals (HMs). These donors are of utmost importance in facilitating the plant development, stabilization of physiological and biochemical processes, and augmentation of anti-oxidative metabolic pathways. Furthermore, the review delves into the interaction between different growth regulators and endogenous hydrogen sulfide and their contributions to mitigating metal-induced phytotoxicity.

Synergistic regulation of hydrogen sulfide and nitric oxide on biochemical components, exopolysaccharides, and nitrogen metabolism in nickel stressed rice field cyanobacteria

The present study examined the regulatory mechanism of hydrogen sulfide (H2S) and nitric oxide (NO) in nickel (Ni) stressed cyanobacteria viz., Nostoc muscorum and Anabaena sp. by analyzing growth, photosynthetic pigments, biochemical components (protein and carbohydrate), exopolysaccharides (EPS), inorganic nitrogen content, and activity of enzymes comprised in nitrogen metabolism and Ni accumulation. The 1 µM Ni substantially diminished growth by 18% and 22% in N. muscorum and Anabaena sp. respectively, along with declining the pigment contents (Chl a/Car ratio and phycobiliproteins), and biochemical components. It also exerted negative impacts on inorganic uptake of nitrate and nitrite contents; nitrate reductase and nitrite reductase; and ammonium assimilating enzymes (glutamine synthetase, glutamate synthase, and glutamate dehydrogenase exhibited a reverse trend) activities. Nonetheless, the adverse impact of Ni can be mitigated through the exogenous supplementation of NaHS [sodium hydrosulfide (8 µM); H2S donor] and SNP [sodium nitroprusside (10 µM); NO donor] which showed substantial improvement on growth, pigments, nitrogen metabolism, and EPS layer and noticeably occurred as a consequence of a substantial reduction in Ni accumulation content which minimized the toxicity effects. The accumulation of Ni on both the cyanobacterial cell surface (EPS layer) are confirmed by the SEM-EDX analysis. Further, the addition of NO scavenger (PTIO; 20 µM) and inhibitor of NO (L-NAME; 100 µM); and H2S scavenger (HT; 20 µM) and H2S inhibitor (PAG; 50 µM) reversed the positive responses of H2S and NO and damages were more prominent under Ni stress thereby, suggesting the downstream signaling of H2S on NO-mediated alleviation. Thus, this study concludes the crosstalk mechanism of H2S and NO in the mitigation of Ni-induced toxicity in rice field cyanobacteria.

Evolution of reactive oxygen species cellular targets for plant development

Reactive oxygen species (ROS) are the key players in regulating developmental processes of plants. Plants have evolved a large array of gene families to facilitate the ROS-regulated developmental process in roots and leaves. However, the cellular targets of ROS during plant evolutionary development are still elusive. Here, we found early evolution and large expansions of protein families such as mitogen-activated protein kinases (MAPK) in the evolutionarily important plant lineages. We review the recent advances in interactions among ROS, phytohormones, gasotransmitters, and protein kinases. We propose that these signaling molecules act in concert to maintain cellular ROS homeostasis in developmental processes of root and leaf to ensure the fine-tuning of plant growth for better adaptation to the changing climate.

Adaptive responses of nitric oxide (NO) and its intricate dialogue with phytohormones during salinity stress

Nitric oxide (NO) is a gaseous free radical that acts as a messenger for various plant phenomena corresponding to photomorphogenesis, fertilisation, flowering, germination, growth, and productivity. Recent developments have suggested the critical role of NO in inducing adaptive responses in plants during salinity. NO minimises salinity-induced photosynthetic damage and improves plant-water relation, nutrient uptake, stomatal conductance, electron transport, and ROS and antioxidant metabolism. NO contributes active participation in ABA-mediated stomatal regulation. Similar crosstalk of NO with other phytohormones such as auxins (IAAs), gibberellins (GAs), cytokinins (CKs), ethylene (ET), salicylic acid (SA), strigolactones (SLs), and brassinosteroids (BRs) were also observed. Additionally, we discuss NO interaction with other gaseous signalling molecules such as reactive oxygen species (ROS) and reactive sulphur species (RSS). Conclusively, the present review traces critical events in NO-induced morpho-physiological adjustments under salt stress and discusses how such modulations upgrade plant resilience.

Aquaporins: a vital nexus in H2O2-gasotransmitter signaling

Land plants have evolved with a complex mechanism of water uptake facilitated by the activity of aquaporins under normal and challenging environments. However, we lack a clear understanding of its interactions with reactive oxygen species, particularly hydrogen peroxide (H2O2) and the gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S), under oxidative stress. Here, we assess the crosstalk of aquaporin function, H2O2 homeostasis, and NO-H2S signaling in plants and provide a computational prediction of cysteine-based oxidative post-translational modifications (oxiPTMs) in plant aquaporins. We propose that aquaporin activity could be regulated by three major oxiPTMs, S-nitrosation, S-sulfenylation, and persulfidation, mediated by NO, H2O2, and H2S, respectively. Therefore, aquaporins might be key players in the gasotransmitter-mediated long-distance oxidative stress signals in plant cells.

H2 O2 , NO, and H2 S networks during root development and signalling under physiological and challenging environments: Beneficial or toxic?

Hydrogen peroxide (H₂ O₂ ) is a reactive oxygen species (ROS) and a key modulator of the development and architecture of the root system under physiological and adverse environmental conditions. Nitric oxide (NO) and hydrogen sulphide (H₂ S) also exert myriad functions on plant development and signalling. Accumulating pieces of evidence show that depending upon the dose and mode of applications, NO and H₂ S can have synergistic or antagonistic actions in mediating H₂ O₂ signalling during root development. Thus, H₂ O₂ -NO-H₂ S crosstalk might essentially impart tolerance to elude oxidative stress in roots. Growth and proliferation of root apex involve crucial orchestration of NO and H₂ S-mediated ROS signalling which also comprise other components including mitogen-activated protein kinase, cyclins, cyclin-dependent kinases, respiratory burst oxidase homolog (RBOH), and Ca²⁺ flux. This assessment provides a comprehensive update on the cooperative roles of NO and H₂ S in modulating H₂ O₂ homoeostasis during root development, abiotic stress tolerance, and root-microbe interaction. Furthermore, it also analyses the scopes of some fascinating future investigations associated with strigolactone and karrikins concerning H₂ O₂ -NO-H₂ S crosstalk in plant roots.

Functions of Melatonin during Postharvest of Horticultural Crops

Melatonin, a tryptophan-derived molecule, is endogenously generated in animal, plant, fungal and prokaryotic cells. Given its antioxidant properties, it is involved in a myriad of signaling functions associated with various aspects of plant growth and development. In higher plants, melatonin (Mel) interacts with plant regulators such as phytohormones, as well as reactive oxygen and nitrogen species including hydrogen peroxide (H2O2), nitric oxide (NO) and hydrogen sulfide (H2S). It shows great potential as a biotechnological tool to alleviate biotic and abiotic stress, to delay senescence and to conserve the sensory and nutritional quality of postharvest horticultural products which are of considerable economic importance worldwide. This review provides a comprehensive overview of the biochemistry of Mel, whose endogenous induction and exogenous application can play an important biotechnological role in enhancing the marketability and hence earnings from postharvest horticultural crops.

Auxin Crosstalk with Reactive Oxygen and Nitrogen Species in Plant Development and Abiotic Stress

The phytohormone auxin acts as an important signaling molecule having regulatory functions during the growth and development of plants. Reactive oxygen species (ROS) are also known to perform signaling functions at low concentrations; however, over-accumulation of ROS due to various environmental stresses damages the biomolecules and cell structures and leads to cell death, and therefore, it can be said that ROS act as a double-edged sword. Nitric oxide (NO), a gaseous signaling molecule, performs a wide range of favorable roles in plants. NO displays its positive role in photomorphogenesis, root growth, leaf expansion, seed germination, stomatal closure, senescence, fruit maturation, mitochondrial activity and metabolism of iron. Studies have revealed the early existence of these crucial molecules during evolution. Moreover, auxin, ROS and NO together show their involvement in various developmental processes and abiotic stress tolerance. Redox signaling is a primary response during exposure of plants to stresses and shows a link with auxin signaling. This review provides updated information related to crosstalk between auxin, ROS and NO starting from their evolution during early Earth periods and their interaction in plant growth and developmental processes as well as in the case of abiotic stresses to plants.

Functions of NO and H2S Signal Molecules Against Plant Abiotic Stress

Nitric oxide (NO) and hydrogen sulfide (H₂S) are two recognized signal molecules in higher plants involved in a wide range of physiological processes and the mechanisms of response against adverse environmental conditions. These molecules can interact to provide an adequate response to palliate the negative impact exerted by stressful conditions, particularly by regulating key components of the metabolism of reactive oxygen species (ROS) to avoid their overproduction and further oxidative damage which, finally, affects cellular functioning. NO and H₂S can exert the regulation over the function of susceptible proteins by posttranslational modifications (PTMs) including nitration, S-nitrosation, and persulfidation but also through the regulation of gene expression by the induction of specific transcription factors which modulate the expression of genes encoding proteins related to stress resistance. This chapter encompasses a wide perspective of the signaling and functional relationships between NO and H₂S to modulate the overproduction of reactive oxygen species, particularly under abiotic stress conditions.

Analysis of Plant L-Cysteine Desulfhydrase (LCD) Isozymes by Non-denaturing Polyacrylamide Gel Electrophoresis

Hydrogen sulfide (H₂S) is a signaling molecule that achieves different regulatory functions in animal and plant cells. The cytosolic enzyme L-cysteine desulfhydrase (LCD; EC 4.4.1.28) catalyzes the conversion of cysteine (L-Cys) to pyruvate and ammonium with the concomitant generation of H₂S, this enzyme being considered one of the main sources of H₂S in higher plants. Using non-denaturing polyacrylamide gel electrophoresis (PAGE) in combination with a specific assay for LCD activity, the present protocol allows identifying diverse LCD isozymes present in different organs (roots, shoots, leaves, and fruits) and plant species including pea, garlic, Arabidopsis, and pepper.

Melatonin-mediated temperature stress tolerance in plants

Global climate changes cause extreme temperatures and a significant reduction in crop production, leading to food insecurity worldwide. Temperature extremes (including both heat and cold stresses) is one of the most limiting factors in plant growth and development and severely affect plant physiology, biochemical, and molecular processes. Biostimulants like melatonin (MET) have a multifunctional role that acts as a "defense molecule" to safeguard plants against the noxious effects of temperature stress. MET treatment improves plant growth and temperature tolerance by improving several defense mechanisms. Current research also suggests that MET interacts with other molecules, like phytohormones and gaseous molecules, which greatly supports plant adaptation to temperature stress. Genetic engineering via overexpression or CRISPR/Cas system of MET biosynthetic genes uplifts the MET levels in transgenic plants and enhances temperature stress tolerance. This review highlights the critical role of MET in plant production and tolerance against temperature stress. We have documented how MET interacts with other molecules to alleviate temperature stress. MET-mediated molecular breeding would be great potential in helping the adverse effects of temperature stress by creating transgenic plants.

Biological Functions of Hydrogen Sulfide in Plants

Hydrogen sulfide (H₂S), which is a gasotransmitter, can be biosynthesized and participates in various physiological and biochemical processes in plants. H₂S also positively affects plants' adaptation to abiotic stresses. Here, we summarize the specific ways in which H₂S is endogenously synthesized and metabolized in plants, along with the agents and methods used for H₂S research, and outline the progress of research on the regulation of H₂S on plant metabolism and morphogenesis, abiotic stress tolerance, and the series of different post-translational modifications (PTMs) in which H₂S is involved, to provide a reference for future research on the mechanism of H₂S action.

Reactive Oxygen, Nitrogen, and Sulfur Species (RONSS) as a Metabolic Cluster for Signaling and Biostimulation of Plants: An Overview

This review highlights the relationship between the metabolism of reactive oxygen species (ROS), reactive nitrogen species (RNS), and H₂S-reactive sulfur species (RSS). These three metabolic pathways, collectively termed reactive oxygen, nitrogen, and sulfur species (RONSS), constitute a conglomerate of reactions that function as an energy dissipation mechanism, in addition to allowing environmental signals to be transduced into cellular information. This information, in the form of proteins with posttranslational modifications or signaling metabolites derived from RONSS, serves as an inducer of many processes for redoxtasis and metabolic adjustment to the changing environmental conditions to which plants are subjected. Although it is thought that the role of reactive chemical species was originally energy dissipation, during evolution they seem to form a cluster of RONSS that, in addition to dissipating excess excitation potential or reducing potential, also fulfils essential signaling functions that play a vital role in the stress acclimation of plants. Signaling occurs by synthesizing many biomolecules that modify the activity of transcription factors and through modifications in thiol groups of enzymes. The result is a series of adjustments in plants' gene expression, biochemistry, and physiology. Therefore, we present an overview of the synthesis and functions of the RONSS, considering the importance and implications in agronomic management, particularly on the biostimulation of crops.

Influence of straw-derived humic acid-like substance on the availability of Cd/As in paddy soil and their accumulation in rice grain

Humic acid amendments have been widely advocated for the remediation of heavy metal-contaminated soil. However, the impacts of straw-derived humic acid-like substances on the remediation of cadmium (Cd) and arsenic (As) co-contaminated paddy soil remain unclear and the potential mechanism required clarification. In this study, we employed a pot experiment and chose a straw-derived humic acid-like substance (BFA) as the amendment with four doses to investigate how BFA affects the availability of Cd and As in soil and their accumulation in rice. The results showed that grain Cd decreased by 25.65-36.03%, while there was no significant change in total As (TAs) with the addition of BFA. The contents of DCB-Fe, DCB-As and DCB-Cd on the root surface decreased by 6.07-40.54% during the whole growth stage. The addition of BFA significantly decreased the pe + pH and enhanced the transformation of crystalline iron oxides (Fed) into amorphous forms (Feo) in the soil. The CaCl₂-extractable Cd decreased and the KH₂PO₄-extractable As increased with the decrease in pe + pH and Fed and the relative increase in Feo. The correlation analysis showed that the decrease in availability of Cd and translocation factor of Cd effectively decreased the grain Cd and the decrease in DCB-Cd may also contribute to decreasing the uptake of Cd by rice. However, the increase in As of roots and shoots might play key roles in restricting the transport of As to rice grains. Consequently, the addition of BFA could effectively reduce the Cd accumulation in rice under flooding conditions, while no risk of As accumulation in rice grain was observed. The present work provides a new perspective for the application of straw-derived humic acid-like substances as amendments on Cd-As co-contaminated soils, which should be advocated as an eco-friendly, economical and effective soil amendment in the future.

Hydrogen sulphide (H2 S) in the hidden half: Role in root growth, stress signalling and rhizospheric interactions

Apart from nitric oxide (NO) and carbon monoxide (CO), hydrogen sulphide (H₂ S) has emerged as a potential gasotransmitter that has regulatory roles in root differentiation, proliferation and stress signalling. H₂ S metabolism in plants exhibits spatio-temporal differences that are intimately associated with sulphide signalling in the cytosol and other subcellular components, e.g. chloroplast and mitochondria. H₂ S biosynthesis in plant organs uses both enzymatic and non-enzymatic pathways. H₂ S generation in roots and aerial organs is modulated by developmental phase and changes in environmental stimuli. H₂ S has an influential role in root development and in the nodulation process. Studies have revealed that H₂ S is a part of the auxin and NO signalling pathways in roots, which induce lateral root formation. At the molecular level, exogenous application of H₂ S regulates expression of several transcription factors, viz. LBD (Lateral organ Boundaries Domain), MYB (myeloblastosis) and AP2/ERF (Apetala 2/ Ethylene Response Factor), which stimulate upregulation of PpLBD16 (Lateral organ boundaries domain 16), thereby significantly increasing the number of lateral roots. Concomitantly, H₂ S acts as a crucial signalling molecule in roots during various abiotic stresses, e.g. drought, salinity heavy metals (HMs), etc., and augments stress tolerance in plants. Interestingly, extensive crosstalk exists between H₂ S, NO, ABA, calcium and ethylene during stress, which escalate plant defence and regulate plant growth and productivity. Hence, the present review will elaborate the role of H₂ S in root development, stress alleviation, legume-Rhizobium symbiosis and rhizosphere signalling. The review also examines the mechanism of H₂ S-mediated abiotic stress mitigation and cross-talk with other signaling molecules.

Hydrogen sulfide: an emerging component against abiotic stress in plants

As a result of climate change, abiotic stresses are the most common cause of crop losses worldwide. Abiotic stresses significantly impair plants' physiological, biochemical, molecular and cellular mechanisms, limiting crop productivity under adverse climate conditions. However, plants can implement essential mechanisms against abiotic stressors to maintain their growth and persistence under such stressful environments. In nature, plants have developed several adaptations and defence mechanisms to mitigate abiotic stress. Moreover, recent research has revealed that signalling molecules like hydrogen sulfide (H₂ S) play a crucial role in mitigating the adverse effects of environmental stresses in plants by implementing several physiological and biochemical mechanisms. Mainly, H₂ S helps to implement antioxidant defence systems, and interacts with other molecules like nitric oxide (NO), reactive oxygen species (ROS), phytohormones, etc. These molecules are well-known as the key players that moderate the adverse effects of abiotic stresses. Currently, little progress has been made in understanding the molecular basis of the protective role of H₂ S; however, it is imperative to understand the molecular basis using the state-of-the-art CRISPR-Cas gene-editing tool. Subsequently, genetic engineering could provide a promising approach to unravelling the molecular basis of stress tolerance mediated by exogenous/endogenous H₂ S. Here, we review recent advances in understanding the beneficial roles of H₂ S in conferring multiple abiotic stress tolerance in plants. Further, we also discuss the interaction and crosstalk between H₂ S and other signal molecules; as well as highlighting some genetic engineering-based current and future directions.

Insights into the beneficial roles of dark septate endophytes in plants under challenging environment: resilience to biotic and abiotic stresses

Dark septate endophytes (DSE) exert a plethora of effects in regulating plant growth, signalling and stress tolerance. The advent of metagenomics has led to the identification of various species of DSE to be associated with plant organs. They are known to modulate growth, nutrient uptake, phytohormone biosynthesis and production of active bioconstituents in several plants. The interactions between the DSE and host plants are mostly mutualistic but they can also be neutral or exhibit negative interactions. The DSE has beneficial role in removal/sequestration of toxic heavy metals from various environmental sites. Here, we discuss the beneficial role of DSE in enhancing plant tolerance to heavy metal stress, drought conditions, high salinity and protection from various plant pathogens. Furthermore, the underlying mechanism of stress resilience facilitated by DSE-plant interaction has also been discussed. The article also provides insights to some important future perspectives associated with DSE-mediated phytoremediation and reclamation of polluted land worldwide thus facilitating sustainable agriculture.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

Nitric oxide donor, sodium nitroprusside modulates hydrogen sulfide metabolism and cysteine homeostasis to aid the alleviation of chromium toxicity in maize seedlings (Zea mays L.)

The current research aimed to assess the protective role of nitric oxide (NO) against chromium (Cr) toxicity in maize seedlings. Chromium (200 µM) lowered osmotic potential in epicotyls and mostly in radicles (by 38% and 63%, respectively) as compared to the control. Sodium nitroprusside (SNP, NO donor) restored seedling biomass (+90% for both organs) and water potential, whereas application of Nω-nitro-L-arginine methylester (_L-NAME, a NOS inhibitor) increased sensitivity to Cr. SNP suppressed Cr-triggered proline accumulation by inhibiting Δ¹-pyrroline-5-carboxylate synthetase activity and stimulating proline dehydrogenase activity, leading to glutamate over-accumulation (~30% for both organs). Cr stimulated cysteine metabolism and this was further enhanced by SNP which stimulated serine acetyl-transferase and O-acetylserine (thiol) lyase activities. This was followed by an increase in endogenous hydrogen sulfide (H₂S) generation by up-regulating L-cysteine desulfhydrase (+205%), D-cysteine desulfhydrase (+150%) and cyanoalanine synthase (+65%) activities in radicles compared to Cr-treatments plants. These positive effects were reduced in _L-NAME compared to control. Combined Cr+SNP affected the levels of compounds involved in glutathione metabolism (γ-glutamyl-cysteinyl, γ-glutamyl-cysteinyl-clycine, γ-cysteinyl-glycine, and glycine.). All together, our findings indicate that NO and elicited cellular H₂S act synergistically to alleviate Cr stress in maize seedlings by influencing a metabolic interplay between cysteine, proline, and glutathione.

Nitric oxide is involved in hydrogen sulfide-induced adventitious rooting in tomato (Solanum lycopersicum)

Nitric oxide (NO) and hydrogen sulfide (H2 S) are signalling molecules that regulate adventitious rooting in plants. However, little is known about the cross-talk between NO and H2 S during adventitious rooting. Tomato (Solanum lycopersicum L.) explants were used to investigate the roles of and relationships between NO and H2 S during rooting. Effects of the NO donor sodium nitroprusside (SNP) and the H2 S donor sodium hydrosulfide (NaHS) on adventitious rooting were dose-dependent, and the greatest biological responses were observed under 25μM SNP and 50μM NaHS. The positive effect of NaHS was reversed by the NO scavenger 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), indicating that the H2 S-induced response was partially NO-dependent. Peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD) activities significantly increased by SNP and NaHS treatment, and indoleacetic acid oxidase (IAAO) activity and the O2 - and H2 O2 content significantly decreased by SNP and NaHS treatment. SNP and NaHS treatment also increased the content of soluble sugar and protein and indole-3-acetic acid (IAA). cPTIO significantly mitigated the increases in POD, PPO and SOD activity and soluble sugar, protein and IAA content induced by NaHS. SNP and NaHS upregulated the expression of auxin-related genes (ARF4 and ARF16 ), cell cycle-related genes (CYCD3 , CYCA3 and CDKA1 ), and antioxidant-related genes (TPX2 , SOD and POD ); whereas cPTIO significantly inhibited the increase in the expression of these genes induced by NaHS. Overall, these results show that NO may be involved in H2 S-induced adventitious rooting by regulating the activity of rooting-related enzymes, the expression of related genes, and the content of various nutrients.

Stress-regulated elements in Lotus spp., as a possible starting point to understand signalling networks and stress adaptation in legumes

Although legumes are of primary economic importance for human and livestock consumption, the information regarding signalling networks during plant stress response in this group is very scarce. Lotus japonicus is a major experimental model within the Leguminosae family, whereas L. corniculatus and L. tenuis are frequent components of natural and agricultural ecosystems worldwide. These species display differences in their perception and response to diverse stresses, even at the genotype level, whereby they have been used in many studies aimed at achieving a better understanding of the plant stress-response mechanisms. However, we are far from the identification of key components of their stress-response signalling network, a previous step for implementing transgenic and editing tools to develop legume stress-resilient genotypes, with higher crop yield and quality. In this review we scope a body of literature, highlighting what is currently known on the stress-regulated signalling elements so far reported in Lotus spp. Our work includes a comprehensive review of transcription factors chaperones, redox signals and proteins of unknown function. In addition, we revised strigolactones and genes regulating phytochelatins and hormone metabolism, due to their involvement as intermediates in several physiological signalling networks. This work was intended for a broad readership in the fields of physiology, metabolism, plant nutrition, genetics and signal transduction. Our results suggest that Lotus species provide a valuable information platform for the study of specific protein-protein (PPI) interactions, as a starting point to unravel signalling networks underlying plant acclimatation to bacterial and abiotic stressors in legumes. Furthermore, some Lotus species may be a source of genes whose regulation improves stress tolerance and growth when introduced ectopically in other plant species.

Interplay between gasotransmitters and potassium is a K+ey factor during plant response to abiotic stress

Carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters known for their roles in plant response to (a)biotic stresses. The crosstalk between these gasotransmitters and potassium ions (K⁺) has received considerable attention in recent years, particularly due to the dual role of K⁺ as an essential mineral nutrient and a promoter of plant tolerance to abiotic stress. This review brings together what it is known about the interplay among NO, CO, H₂S and K⁺ in plants with focus on the response to high salinity. Some findings obtained for plants under water deficit and metal stress are also presented and discussed since both abiotic stresses share similarities with salt stress. The molecular targets of the gasotransmitters NO, CO and H₂S in root and guard cells that drive plant tolerance to salt stress are highlighted as well.

Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning

Nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters, which are involved in almost all plant physiological and stress-related processes. With its antioxidant regulatory properties, NO on its own ameliorates plant stress, while H₂S, a foul-smelling gas, has differential effects. Recent studies have shown that these signaling molecules are involved in intertwined pathway networks. This is due to the contrasting effects of NO and H₂S depending on cell type, subcellular compartment, and redox status, as well as the flux and dosage of NO and H₂S in different plant species and cellular contexts. Here, we provide a comprehensive review of the complex networks of these molecules, with particular emphasis on root development, stomatal movement, and plant cell death.

An Updated Insight Into Molecular Mechanism of Hydrogen Sulfide in Cardiomyopathy and Myocardial Ischemia/Reperfusion Injury Under Diabetes

Cardiovascular diseases are the most common complications of diabetes, and diabetic cardiomyopathy is a major cause of people death in diabetes. Molecular, transcriptional, animal, and clinical studies have discovered numerous therapeutic targets or drugs for diabetic cardiomyopathy. Within this, hydrogen sulfide (H₂S), an endogenous gasotransmitter alongside with nitric oxide (NO) and carbon monoxide (CO), is found to play a critical role in diabetic cardiomyopathy. Recently, the protective roles of H₂S in diabetic cardiomyopathy have attracted enormous attention. In addition, H₂S donors confer favorable effects in myocardial infarction, ischaemia-reperfusion injury, and heart failure under diabetic conditions. Further studies have disclosed that multiplex molecular mechanisms are responsible for the protective effects of H₂S against diabetes-elicited cardiac injury, such as anti-oxidative, anti-apoptotic, anti-inflammatory, and anti-necrotic properties. In this review, we will summarize the current findings on H₂S biology and pharmacology, especially focusing on the novel mechanisms of H₂S-based protection against diabetic cardiomyopathy. Also, the potential roles of H₂S in diabetes-aggravated ischaemia-reperfusion injury are discussed.

Hydrogen Sulfide in Plants: Crosstalk with Other Signal Molecules in Response to Abiotic Stresses

Hydrogen sulfide (H₂S) has recently been considered as a crucial gaseous transmitter occupying extensive roles in physiological and biochemical processes throughout the life of plant species. Furthermore, plenty of achievements have been announced regarding H₂S working in combination with other signal molecules to mitigate environmental damage, such as nitric oxide (NO), abscisic acid (ABA), calcium ion (Ca²⁺), hydrogen peroxide (H₂O₂), salicylic acid (SA), ethylene (ETH), jasmonic acid (JA), proline (Pro), and melatonin (MT). This review summarizes the current knowledge within the mechanism of H₂S and the above signal compounds in response to abiotic stresses in plants, including maintaining cellular redox homeostasis, exchanging metal ion transport, regulating stomatal aperture, and altering gene expression and enzyme activities. The potential relationship between H₂S and other signal transmitters is also proposed and discussed.

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Bacterial volatile compound-based tools for crop management and quality

Bacteria produce a huge diversity of metabolites, many of which mediate ecological relations. Among these, volatile compounds cause broad-range effects at low doses and, therefore, may be exploited for plant defence strategies and agricultural production, but such applications are still in their early development. Here, we review the latest technologies involving the use of bacterial volatile compounds for phytosanitary inspection, biological control, plant growth promotion, and crop quality. We highlight a variety of effects with a potential applicative interest, based on either live biocontrol and/or biostimulant agents, or the isolated metabolites responsible for the interaction with hosts or competitors. Future agricultural technologies may benefit from the development of new analytical tools to understand bacterial interactions with the environment.

Nitric oxide, crosstalk with stress regulators and plant abiotic stress tolerance

Nitric oxide is a dynamic gaseous molecule involved in signalling, crosstalk with stress regulators, and plant abiotic-stress responses. It has great exploratory potentials for engineering abiotic stress tolerance in crops. Nitric oxide (NO), a redox-active gaseous signalling molecule, though present uniformly through the eukaryotes, maintain its specificity in plants with respect to its formation, signalling, and functions. Its cellular concentrations are decisive for its function, as a signalling molecule at lower concentrations, but triggers nitro-oxidative stress and cellular damage when produced at higher concentrations. Besides, it also acts as a potent stress alleviator. Discovered in animals as neurotransmitter, NO has come a long way to being a stress radical and growth regulator in plants. As a key redox molecule, it exhibits several key cellular and molecular interactions including with reactive chemical species, hydrogen sulphide, and calcium. Apart from being a signalling molecule, it is emerging as a key player involved in regulations of plant growth, development and plant-environment interactions. It is involved in crosstalk with stress regulators and is thus pivotal in these stress regulatory mechanisms. NO is getting an unprecedented attention from research community, being investigated and explored for its multifaceted roles in plant abiotic stress tolerance. Through this review, we intend to present the current knowledge and updates on NO biosynthesis and signalling, crosstalk with stress regulators, and how biotechnological manipulations of NO pathway are leading towards developing transgenic crop plants that can withstand environmental stresses and climate change. The targets of various stress responsive miRNA signalling have also been discussed besides giving an account of current approaches used to characterise and detect the NO.

Regulatory hubs and strategies for improving heavy metal tolerance in plants: Chemical messengers, omics and genetic engineering

Heavy metal (HM) accumulation in the agricultural soil and its toxicity is a major threat for plant growth and development. HMs disrupt functional integrity of the plants, induces altered phenological and physiological responses and slashes down qualitative crop yield. Chemical messengers such as phytohormones, plant growth regulators and gasotransmitters play a crucial role in regulating plant growth and development under metal toxicity in plants. Understanding the intricate network of these chemical messengers as well as interactions of genes/metabolites/proteins associated with HM toxicity in plants is necessary for deciphering insights into the regulatory circuit involved in HM tolerance. The present review describes (a) the role of chemical messengers in HM-induced toxicity mitigation, (b) possible crosstalk between phytohormones and other signaling cascades involved in plants HM tolerance and (c) the recent advancements in biotechnological interventions including genetic engineering, genome editing and omics approaches to provide a step ahead in making of improved plant against HM toxicities.

Exogenous Potassium (K+) Positively Regulates Na+/H+ Antiport System, Carbohydrate Metabolism, and Ascorbate-Glutathione Cycle in H2S-Dependent Manner in NaCl-Stressed Tomato Seedling Roots

Potassium (K⁺) is one of the vital macronutrients required by plants for proper growth and blossoming harvest. In addition, K⁺ also plays a decisive role in promoting tolerance to various stresses. Under stressful conditions, plants deploy their defense system through various signaling molecules, including hydrogen sulfide (H₂S). The present investigation was carried out to unravel the role of K⁺ and H₂S in plants under NaCl stress. The results of the study show that NaCl stress caused a reduction in K⁺ and an increase in Na⁺ content in the tomato seedling roots which coincided with a lower H⁺-ATPase activity and K⁺/Na⁺ ratio. However, application of 5 mM K⁺, in association with endogenous H₂S, positively regulated the Na⁺/H⁺ antiport system that accelerated K+ influx and Na+ efflux, resulting in the maintenance of a higher K⁺/Na⁺ ratio. The role of K⁺ and H₂S in the regulation of the Na⁺/H⁺ antiport system was validated by applying sodium orthovanadate (plasma membrane H⁺-ATPase inhibitor), tetraethylammonium chloride (K⁺ channel blocker), amiloride (Na⁺/H⁺ antiporter inhibitor), and hypotaurine (HT, H₂S scavenger). Application of 5 mM K⁺ positively regulated the ascorbate–glutathione cycle and activity of antioxidant enzymes that resulted in a reduction in reactive oxygen species generation and associated damage. Under NaCl stress, K⁺ also activated carbohydrate metabolism and proline accumulation that caused improvement in osmotic tolerance and enhanced the hydration level of the stressed seedlings. However, inclusion of the H₂S scavenger HT reversed the effect of K⁺, suggesting H₂S-dependent functioning of K⁺ under NaCl stress. Therefore, the present findings report that K⁺, in association with H₂S, alleviates NaCl-induced impairments by regulating the Na⁺/H⁺ antiport system, carbohydrate metabolism, and antioxidative defense system.

Nitric oxide (NO) and salicylic acid (SA): A framework for their relationship in plant development under abiotic stress

The free radical nitric oxide (NO) and the phenolic phytohormone salicylic acid (SA) are signal molecules which exert key functions at biochemical and physiological levels. Abiotic stresses, especially in early plant development, impose the biggest threats to agricultural systems and crop yield. These stresses impair plant growth and subsequently cause a reduction in root development, affecting nutrient uptake and crop productivity. The molecules NO and SA have been identified as robust tools for efficiently mitigating the negative effects of abiotic stress in plants. SA is engaged in an array of tasks under adverse environmental situations. The function of NO depends on its cellular concentration; at a low level, it acts as a signal molecule, while at a high level, it triggers nitro-oxidative stress. The crosstalk between NO and SA involving different signalling molecules and regulatory factors modulate plant function during stressful situations. Crosstalk between these two signalling molecules induces plant tolerance to abiotic stress and needs further investigation. This review aims to highlight signalling aspects of NO and SA in higher plants and critically discusses the roles of these two molecules in alleviating abiotic stress.

Emerging warriors against salinity in plants: Nitric oxide and hydrogen sulphide

The agriculture sector is vulnerable to various environmental stresses, which significantly affect plant growth, performance, and development. Abiotic stresses, such as salinity and drought, cause severe losses in crop productivity worldwide. Soil salinity is a major stress suppressing plant development through osmotic stress accompanied by ion toxicity, nutritional imbalance, and oxidative stress. Various defense mechanisms like osmolytes accumulations, activation of stress-induced genes, and transcription factors, production of plant growth hormones, accumulation of antioxidants, and redox defense system in plants are responsible for combating salt stress. Nitric oxide (NO) and hydrogen sulphide (H₂ S) have emerged as novel bioactive gaseous signaling molecules that positively impact seed germination, homeostasis, plant metabolism, growth, and development, and are involved in several plant acclimation responses to impart stress tolerance in plants. NO and H₂ S trigger cell signaling by activating a cascade of biochemical events that result in plant tolerance to environmental stresses. NO- and H₂ S-mediated signaling networks, interactions, and crosstalks facilitate stress tolerance in plants. Research on the roles and mechanisms of NO and H₂ S as challengers of salinity is entering an exponential exploration era. The present review focuses on the current knowledge of the mechanisms of stress tolerance in plants and the role of NO and H₂ S in adaptive plant responses to salt stress and provides an overview of the signaling mechanisms and interplay of NO and H₂ S in the regulation of growth and development as well as modulation of defense responses in plants and their long term priming effects for imparting salinity tolerance in plants.

NO Network for Plant-Microbe Communication Underground: A Review

Mechanisms governing plant-microbe interaction in the rhizosphere attracted a lot of investigative attention in the last decade. The rhizosphere is not simply a source of nutrients and support for the plants; it is rather an ecosystem teeming with diverse flora and fauna including different groups of microbes that are useful as well as harmful for the plants. Plant-microbe interaction occurs via a highly complex communication network that involves sophisticated machinery for the recognition of friend and foe at both sides. On the other hand, nitric oxide (NO) is a key, signaling molecule involved in plant development and defense. Studies on legume-rhizobia symbiosis suggest the involvement of NO during recognition, root hair curling, development of infection threads, nodule development, and nodule senescence. A similar role of NO is also suggested in the case of plant interaction with the mycorrhizal fungi. Another, insight into the plant-microbe interaction in the rhizosphere comes from the recognition of pathogen-associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs) by the host plant and thereby NO-mediated activation of the defense signaling cascade. Thus, NO plays a major role in mediating the communication between plants and microbes in the rhizosphere. Interestingly, reports suggesting the role of silicon in increasing the number of nodules, enhancing nitrogen fixation, and also the combined effect of silicon and NO may indicate a possibility of their interaction in mediating microbial communication underground. However, the exact role of NO in mediating plant-microbe interaction remains elusive. Therefore, understanding the role of NO in underground plant physiology is very important, especially in relation to the plant's interaction with the rhizospheric microbiome. This will help devise new strategies for protection against phytopathogens and enhancing plant productivity by promoting symbiotic interaction. This review focuses on the role of NO in plant-microbe communication underground.

Loss of function of the chloroplast membrane K+/H+ antiporters AtKEA1 and AtKEA2 alters the ROS and NO metabolism but promotes drought stress resilience

Potassium (K⁺) exerts key physiological functions such as osmoregulation, stomatal movement, membrane transport, protein synthesis and photosynthesis among others. Previously, it was demonstrated in Arabidopsis thaliana that the loss of function of the chloroplast K⁺Efflux Antiporters KEA1 and KEA2, located in the inner envelope membrane, provokes inefficient photosynthesis. Therefore, the main goal of this study was to evaluate the potential impact of the loss of function of those cation transport systems in the metabolism of reactive oxygen and nitrogen species (ROS and RNS). Using 14-day-old seedlings from Arabidopsis double knock-out kea1kea2 mutants, ROS metabolism and NO content in roots and green cotyledons were studied at the biochemical level. The loss of function of AtKEA1 and AtKEA2 did not cause oxidative stress but it provoked an alteration of the ROS homeostasis affecting some ROS-generating enzymes. These included glycolate oxidase (GOX) and NADPH-dependent superoxide generation activity, enzymatic and non-enzymatic antioxidants and both NADP-isocitrate dehydrogenase and NADP-malic enzyme activities. NO content, analyzed by confocal laser scanning microscopy (CLSM), was negatively affected in both photosynthetic and non-photosynthetic organs in kea1kea2 mutant seedlings. Furthermore, the S-nitrosoglutathione reductase (GSNOR) protein expression and activity were downregulated in kea1kea2 mutants, whereas the tyrosine nitrated protein profile, analyzed by immunoblot, was unaffected but the relative expression of each immunoreactive band changed. Moreover, kea1kea2 mutants showed an increased photorespiratory pathway and stomata closure, thus promoting a higher resilience to drought stress. Data suggest that the chloroplast osmotic balance and integrity maintained by AtKEA1 and AtKEA2 are necessary to keep the balance of ROS/RNS metabolism. Moreover, these data open new questions about how endogenous NO generation might be affected by the K⁺/H⁺ transport located in the chloroplasts.

Regulation of growth in peach roots by exogenous hydrogen sulfide based on RNA-Seq

Hydrogen sulfide (H₂S) has been shown to regulate many physiological processes of plants. In this study, we observed that 0.2 mM sodium hydrosulfide (NaHS), a donor of H₂S, can regulate the root architecture of peach seedlings, increasing the number of lateral roots by 40.63%. To investigate the specific mechanisms by which H₂S regulates root growth in peach, we used RNA sequencing and heterologous expression technology. Our results showed that exogenous H₂S led to a 44.50% increase in the concentration of endogenous auxin. Analyses of differentially expressed genes (DEGs) revealed that 963 and 1113 genes responded to H₂S on days one and five of treatment, respectively. Among the DEGs, 26 genes were involved in auxin biosynthesis, transport, and signal transduction. Using weighted correlation network analysis, we found that the auxin-related genes in the H₂S-specific gene module were disproportionately involved in polar transport, which may play an important role in H₂S-induced root growth. In addition, we observed that the expression of LATERAL ORGAN BOUNDARIES DOMAIN 16 (PpLBD16) was significantly up-regulated by exogenous application of H₂S in peach. Overexpression of PpLBD16 in an Arabidopsis system yielded a 66.83% increase in the number of lateral roots. Under exposure to exogenous H₂S, there was also increased expression of genes related to cell proliferation, indicating that H₂S regulates the growth of peach roots. Our work represents the first comprehensive transcriptomic analysis of the effects of exogenous application of H₂S on the roots of peach, and provides new insights into the mechanisms underlying H₂S-induced root growth.